In a digital radionavigation reception system, the demodulation of the signals is performed in digital mode. Before performing this demodulation, an analogue stage of the receiver reduces the signal to around an intermediate frequency. Another function of this analogue stage is to amplify the signal and to filter it using an anti-aliasing filter. After filtering, the signal is digitized by using an analogue-digital converter, usually designated by the acronym ADC. In order to control the gain of the analogue subsystem so as to have a maximum efficiency of the ADC converter, an adaptive gain control AGC loop, also called automatic gain control loop, is usually used.
Furthermore, for certain applications, the digital radionavigation reception system has to take account of the fact that the analogue signals received include a useful part from which the digital data can be obtained, but also interference. As an example, it is planned for the future GNSS (Global Navigation Satellite System) satellite navigation systems, such as the modernized GPS and the Galileo system, to use frequency bands already used for civil aviation. For example, the band L5 can be used to receive the frequencies of the VOR/DME (Very Omni-Range/Distance Measurement Equipment) systems. To these frequencies, interferences are therefore introduced that can affect the correct operation of GNSS signal reception systems. These interferences consist of pulses which are of short duration, but powerful and frequent, and are able to prevent good reception of the GNSS signals.
In order to reduce the impact of these interferences, a known interference suppression technique, usually referred to as “blanking” can be used. This technique consists in zeroing the signal received when an excessively high power level is detected. This power level reflects the presence of interference pulses, also called pulsed interference. After suppression, only the portions of the signal received without pulsed interference remain, said portions normally being sufficient to demodulate the GNSS signal and enable navigation. Hereinafter in the description, reference is made to this technique by using the expression “temporary signal cancellation”.
In order to implement the temporary signal cancellation technique, a fixed threshold has to be selected on the estimated power level, beyond which the signal is cancelled. The role of the AGC loop is then to maintain an optimum signal level relative to this threshold. The AGC loop usually uses a power measurement as input. When the temporary signal cancellation technique is used, the power measurement for the AGC loop is usually implemented after this processing operation. Thus, the power measurement is not disturbed by the interferences.
One drawback is that the reception system may diverge in the presence of inferences. This is because, when the interferences resulting in a cancellation of the signal are numerous, the average power measured at the input of the AGC loop decreases artificially. This causes the AGC loop to react, said loop increasing the value of the analogue gain ga used to control the power of the signal at the input of the ADC converter. The amplitude of the signal at the input of the module responsible for implementing the temporary signal cancellation is then greater relative to the predefined cancellation threshold. The signal received is then zeroed even more frequently and the AGC loop diverges, rendering the demodulation of the radionavigation signal impossible.